U.S. patent application number 13/465244 was filed with the patent office on 2012-11-15 for burnthrough formulations.
This patent application is currently assigned to APPLIED NANOTECH HOLDINGS, INC.. Invention is credited to Ovadia Abed, Samuel Kim, Yunjun Li, James P. Novak.
Application Number | 20120288991 13/465244 |
Document ID | / |
Family ID | 47139587 |
Filed Date | 2012-11-15 |
United States Patent
Application |
20120288991 |
Kind Code |
A1 |
Abed; Ovadia ; et
al. |
November 15, 2012 |
BURNTHROUGH FORMULATIONS
Abstract
For solar cell fabrication, the addition of precursors to
printable media to assist etching through silicon nitride or
silicon oxide layer thus affording contact with the substance
underneath the nitride or oxide layer. The etching mechanism may be
by molten ceramics formed in situ, fluoride-based etching, as well
as a combination of the two.
Inventors: |
Abed; Ovadia; (Austin,
TX) ; Li; Yunjun; (Austin, TX) ; Novak; James
P.; (Austin, TX) ; Kim; Samuel; (Austin,
TX) |
Assignee: |
APPLIED NANOTECH HOLDINGS,
INC.
Austin
TX
|
Family ID: |
47139587 |
Appl. No.: |
13/465244 |
Filed: |
May 7, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61484039 |
May 9, 2011 |
|
|
|
Current U.S.
Class: |
438/98 ;
106/31.92; 257/E31.124 |
Current CPC
Class: |
Y10S 977/773 20130101;
H01B 1/02 20130101; H01L 31/022425 20130101; Y02E 10/50
20130101 |
Class at
Publication: |
438/98 ;
106/31.92; 257/E31.124 |
International
Class: |
H01L 31/18 20060101
H01L031/18; C09D 11/00 20060101 C09D011/00 |
Claims
1. An ink comprising metal particles, a solvent, a dispersant, and
a low melting point ceramic precursor.
2. The ink as recited in claim 1, wherein the low melting point
ceramic precursor forms a ceramic upon heating, wherein the ceramic
reacts with silicon nitride allowing the metal particles to diffuse
through the silicon nitride.
3. The ink as recited in claim 1, wherein components of the ceramic
precursor are selected from a group consisting of oxides,
hydroxides and fluorides.
4. The ink as recited in claim 3, wherein the oxides, hydroxides,
and fluorides are selected from a group consisting of silicon,
boron, phosphorus, bismuth, zinc, lead, tin, antimony, arsenic,
cerium, copper, silver, indium, beryllium, vanadium, thorium, and
cadmium.
5. The ink as recited in claim 1, wherein the low-melt ceramic
precursor are configured so that upon treatment at an elevated
temperature form oxides, hydroxides, or fluorides.
6. The ink as recited in claim 5, wherein atoms of the precursor
are connected to other atoms by covalent bonds, coordinative bonds,
or salts.
7. The ink as recited in claim 6, wherein the ceramic precursor is
selected from a group consisting of phenylboronic acid,
triisopropyl borate, diphenylborinic acids, boric acid, dibutyltin
oxide, Zn-EDTA complex, lead acetylacetonate, beryllium fluoride,
zinc chloride, lead acetate and tetraethylammonium
tetrafluoroborate.
8. The ink as recited in claim wherein the precursors react with
other oxygen containing materials to form oxides and
hydroxides.
9. The ink as recited in claim 5, wherein the elevated temperature
is between 200.degree. C. and 800.degree. C.
10. The ink as recited in claim 1, wherein the ink is used to form
conductive electrodes on a solar cell.
11. A method for making a solar cell comprising: depositing a metal
ink onto a silicon wafer, wherein the metal ink comprises nickel
nanoparticles and a low melting point ceramic precursor; drying the
metal ink: firing the metal ink to form a conductive electrode
having a conductive contact with the silicon wafer.
12. The method as recited in claim 11, wherein the low melting
point ceramic precursor comprises both boron oxide and the fluoride
salts.
13. The method as recited in claim 11, wherein the firing
temperature is between 200.degree. C. and 800.degree. C.
14. The method as recited in claim 11, wherein the low melting
point ceramic precursor comprises boron oxide.
15. The method as recited in claim 11, wherein the fluoride source
comprises of tetraalkylammonium fluoride salts.
16. The method as recited in claim 11, wherein the
tetraalkylammonium fluoride salts are tetramethylammonium fluoride
(TMF).
17. The method as recited in claim 11, wherein the
tetraalkylammonium fluoride salts are tetraethylammonium fluoride
(TEF).
Description
[0001] This application claims priority to U.S. Provisional
Application Ser. No. 61/484,039, which is hereby incorporated by
reference herein.
TECHNICAL FIELD
[0002] The present invention is related to inks and paste
formulations for the metallization of solar cells in which
"burnthrough" (e.g., penetration through a layer such as silicon
nitride) is utilized to make contact with the materials below.
BACKGROUND AND SUMMARY
[0003] Silicon nitride or silicon oxide layers may he made on
silicon and on other substrates. These layers may perform as
anti-reflecting, passivation, or electronically insulating layers.
By design, the nitride and oxide layers are highly resistant to
material diffusion and are also chemically resistant to most
reactions. Such layers present an obstacle when it is desired to
form direct contact with the substrate underneath them. In some
cases, it is desired to selectively etch specific areas of these
impervious layers as recited in a desired pattern while keeping the
rest of the coating intact. Typical methods for patterned etching
are laser ablation and chemical etching using a photoresist layer
or protective mask. Laser ablation uses a focused laser combined
with a tracking mirror to effectively remove the silicon nitride
layers from the surface. Adjustment of the laser power and
wavelength allows for selective removal of just the top nitride or
oxide layers on top of the silicon. Chemical etching is another
method for surface coating layer removal.
[0004] Consider an example of printing a pattern on nitride-coated
silicon using metal-containing ink or paste with the intention of
forming an electrical contact between the metal and the silicon
underneath the nitride layer. One approach to achieving this goal
is coating the substrate with a protective layer mask as recited in
a predetermined pattern. This mask keeps the desired pattern
exposed: then, the substrate is treated with etching reagents. One
manner for etching these layers involves using wet reagents based
on hydrofluoric acid (HF) and/or phosphoric acid (H.sub.3PO.sub.4).
These reagents may be applied by involve dipping the substrate or
covering it with a film of the liquid reagent. The protective layer
can later he removed. After the desired pattern is etched, it is
possible to print the metal ink on the now-exposed pattern. This
approach involves three steps: the two steps of forming and
removing of the protective layer, and the etching step itself. In
addition, it poses technical requirements of aligning the printer
as recited in the pattern. Another approach is adding the etching
reagent directly into the metallic ink or paste, thus allowing the
reagents in the ink to etch its way through the nitride or oxide
layer. This approach eliminates the use of protective coatings and
a dedicated etching step.
[0005] Currently, this objective may be achieved by adding low
melting point glass frits to the printed media, e.g., addition of
glass frit to a printed media of silver paste composed of silver
particles. After printing the paste, the whole substrate is exposed
to elevated temperatures sufficient to melt the glass frit. The
molten glass frit reacts with the silicon nitride or oxide layer
and allows the silver particles to diffuse and contact the silicon,
layer underneath. The areas that were not printed with the paste
are not etched. The glass frit may act as a flux that helps to etch
the surface of metallic particles in the media and helps joining
them together to increase adhesion.
[0006] The glass frit may be added in the form of powder to the
printed media. This approach has drawbacks. The particle size of
the powder may limit its application or print methods. Small
particle size is required to allow the particles to pass through a
small print nozzle. Typically, the particle diameter in the ink
must be at least 20 times smaller than the inner diameter of the
nozzle or print opening. More preferably, the particle size will be
at least 50 times smaller than the nozzle diameter. In one example,
inkjet printing nozzles may have a diameter of 20 microns. It is
difficult to obtain glass frit materials below 1 micron in
diameter, as the glass frit is manufactured by milling larger sizes
of the glass composition into small particles. As a result, it is
almost impossible to form inks that comprise stable dispersions of
glass frits, and it is not trivial to inkjet such formulations.
[0007] Aspects of the present invention utilize in situ formation
of glass and ceramics to assist a burnthrough of silicon nitride
and silicon oxide layers using oxides, oxide forming precursors, or
mixtures thereof. Embodiments of the present invention produce the
glass frit in situ from soluble or dispersible components, which
upon exposure to elevated temperatures form the desired glass that
etches the nitride or oxide layer, enabling the metallic particles
to reach the silicon substrate underneath the etched layer.
[0008] An advantage of this approach, as reflected from the
previous discussion about the drawbacks of conventional glass
frits, is that the glass composition can be determined by modifying
the glass forming components and proportions. The glass forming
components can he fully dissolved in the media, thus eliminating
problems associated with dispersion. This approach provides a very
flexible tool for designing glass with the optimal properties. This
approach is valid even when the glass forming components are not
soluble in the media.
[0009] Identity of the Active Species Responsible for Etching:
[0010] In either case, whether ready-made glass frit or
glass-forming materials are employed, the nature of the active
species is ambiguous (e.g., not well defined) due to other
components that may be present in the formulations. These include
metal oxides and salts. Such additives can react with the molten
ready-made glass or with the glass-forming materials, thus creating
a new species with different activity and chemical definition. The
active species responsible for etching the nitride or oxide coating
is a relatively low-melting inorganic material, but its identity
may not he necessarily defined as glass. The next paragraphs
further explain this point and show the broad definitions and
interpretations available for glasses and ceramics.
[0011] Glasses are noncrystalline structures, usually consisting of
mixtures of oxides, mainly of silicon, boron, phosphorus,
potassium, sodium, lead, antimony, bismuth as well as other
elements. It is also possible to have glasses that become
crystalline at room temperature, and may not be defined as glasses
under certain terms. Glasses may contain negatively charged
elements other than oxygen, such as in the case of fluorosilicate
and beryllium fluoride-based glasses.
[0012] Ceramics:
[0013] From the Kirk Othmer Encyclopedia of Chemical Technology:
"Ceramics may be defined as a class of inorganic, nonmetallic
solids that are subjected to high temperature in manufacture or
use. Ceramics are distinguished both from metals and metallic
alloys and from organic materials such as polymers and plastics,
and although syntheses may involve solutions or the final products
are solids. The most common ceramics are oxides, carbides (qv), and
nitrides (qv), but suicides, borides, phosphides, sulfides,
tellurides, and selenides are ceramics, as well as elemental
materials such as carbon and silicon. Ceramic synthesis and
processing generally involve high temperatures and the resulting
materials are refractory or heat resistant. Ceramics are commonly
thought to include only polycrystalline materials, but glasses,
which are noncrystalline, and single-crystal materials such as ruby
lasers, are classified as ceramics materials." From the foregoing
definition, glasses fall into the wider group of ceramic
materials.
[0014] Definition of Active Etching Species As Recited in
Embodiments of the Invention:
[0015] As previously noted, the active species responsible for
etching are low melting inorganic materials; however, the identity
of these inorganic species may not be known due to the complexity
of the etching process, which include reactions of the formulation
components between themselves, reactions with the nitride or oxide
coating, as well as reactions with the substrate beneath the
coating. From the previous discussion, it is seen that the
definition of the active species responsible for the etching is
ambiguous (e.g., glass or ceramic). Therefore, in this disclosure,
the glass frits are described as low-melt ceramics, thus defining
aspects of the present invention as low-melt ceramic precursors,
rather than glass-forming components. This definition includes
glasses, as well as low-melting point inorganic materials, which
may not be defined as glasses.
[0016] Low-melt ceramic precursors may be oxides of boron, bismuth,
phosphorus, antimony, arsenic, tin, lead, zinc, cerium, aluminum,
thorium, indium, as well as other elements. Also included are
compounds that decompose to give oxides, hydroxides, or salts upon
treatment at elevated temperatures. Examples include organic
derivatives where the element of interest is covalently connected
to organic structures, such as in boronic acids, boronate esters,
dialkyltin oxides, etc., or by coordinative bond, such as in
zinc-EDTA complex, bismuth-salicylic acid complexes, bismuth
acetylacetonate, etc. Inorganic salts such as beryllium fluoride
having a melting point of 554.degree. C. are included as well.
[0017] Another benefit that arises from using ready-made glass
frits known in the art, or low-melt ceramic precursors as herein
disclosed, is better adhesion of the coating to the substrate,
since the molten ceramics, once cooled and solidified, function as
a binder.
[0018] Another approach according to aspects of the present
invention involves organic fluoride salts and fluorine-containing
polymers to assist burnthrough of silicon nitride and silicon oxide
layers. Certain phosphate and fluoride salts are capable of etching
the nitride or oxide layers. U.S. Pat. No. 7,837,890 describes
formulation of printing media paste using ammonium fluoride
(NH.sub.4F). The rational behind using ammonium fluoride is that it
can decompose to hydrogen fluoride upon treatment at elevated
temperatures
[0019] The drawbacks of ammonium fluoride and also the analogues
ammonium bifluoride (NH.sub.4FHF) salts is that they are soluble
only in water, only slightly soluble in alcohol, and cannot
dissolve in common organic solvents. In order to use them in
organic based formulations, it is necessary to disperse them. This
fact presents a serious obstacle in using these materials in low
viscosity liquids, such as inks, and limits their use to pastes
where the high viscosity assists in forming homogenous dispersions
stable long enough for practical use.
[0020] Herein is disclosed, a fluoride derivative never before
tested. The fluoride derivative is a quaternary ammonium fluoride
salt. The material shown at the example is tetraethylammonium.
fluoride. This material is easily soluble in water as well as in
common organic solvents. This material, when added to a mixture of
nickel nanoparticles and applied on silicon nitride coated silicon,
clearly showed capabilities to etch the nitride coating and form an
electrical contact between the cured nickel film and the silicon
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 illustrates embodiments of the present invention.
[0022] FIG. 2 illustrates a process as recited in embodiments of
the present invention.
DETAILED DESCRIPTION
[0023] Referring to FIG. 1, there is illustrated an example of a
structure of a solar cell 100 in accordance with embodiments of the
present invention. Silicon solar cells use metal paste materials to
create the electrical contacts on the front and backsides. Aluminum
may be used for the p-doped side of the silicon, and silver may he
used for the n-doped silicon. In a p-type solar cell, the p-doped
side may be the back of the wafer, and the n-doped side may be the
top of the wafer.
[0024] The solar cell structure 100 has dimensional limitations on
the diffusion profile. For example, the overall wafer begins with
p-type diffusion. This doping type penetrates the entire wafer 104.
To create the diode junction, only a shallow region (e.g., less
than 1 micron) is u doped to produce the n-type emitter layer 103.
After the diode junction is created by the interface between the p
and n doped regions, the respective sides are metallized to collect
the electron-hole pairs generated by the photovoltaic effect in the
operating solar cell 100.
[0025] Metallization layers may be applied using a printing method,
such as direct screen printing. Other printing methods may he used,
such as inkjet printing, spray printing, and/or aerosolized jet
printing. Each of these printing procedures requires highly
specialized ink or paste materials. A paste or ink material may
comprise metal particles, inorganic components, solvents,
dispersants, and/or vehicles components. Each of these components
may vary in total percentage of the composition and may be tailored
to achieve different functions (e.g., dispersion quality, control
over viscosity, control of surface tension, control over surface
energy and spreading). These types of metallic materials for
printing are referred to as inks, yet the physical requirements of
a particular ink depends on the utilized print or application
method. Screen printing and nozzle dispensing generally utilize
inks having a high viscosity (e.g., >1000 cP) and exhibiting a
thick, paste consistency. Inkjet, spray printing, and aerosolized
jets generally utilize a lower viscosity ink (e.g., <1000 cP).
Nevertheless, embodiments of the present invention may utilize
either inks or pastes.
[0026] Referring to FIG. 2, there are several steps to process the
printed metallization material before it can be considered an
integral part of the solar cell. After placing (e.g., depositing,
printing) the metallization material onto the silicon wafer in step
201, the material is dried in step 202, to remove any residual
solvents and inhibit further spreading. After the drying step, the
metallization paste is fired in step 203 to further remove volatile
additives. This firing melts the sinter particles in the
metallization material into a continuous film and diffuses the
metal in the metallization material into the silicon forming an
ohmic electrical contact. Each type of metallization layer has
different requirements depending on the cell structure or
architecture.
[0027] The same p-type solar cell 100 may have a silver metallic
contact 101 on the top. The top of the cell 100 has a shallow
region 103 that is doped n-type. This material (e.g., silver (Ag))
is deposited directly on top of a silicon nitride layer 102. The
nitride layer 102 serves as a passivation layer and anti-reflective
coating to protect the cell 100 and trap more light, respectively.
The issue with firing this type of cell is that the top layer of
silver is fired at a high temperature such that a reaction between
the glass-frit additives in the paste and the silicon nitride can
occur creating a diffusion channel for the silver metal to make a
contact with the silicon. If the temperature is too low, no
reaction with the nitride will occur and no contact will be formed.
If the temperature is too high, the contact will be formed but the
metal will diffuse rapidly through the n-type doping layer at the
top of the cell. The relative rate steps for the reaction kinetics
are very different. Empirically, the reaction between the glass
frit and the nitride is relatively slow. Once the reaction has
occurred, the diffusion of the silver into the silicon is
relatively fast. If the metal passes the p-n interface junction,
the cell will have poor performance. It is desirable to have the
reaction with the nitride occur, the silver to diffuse through the
nitride and make contact with the silicon, and then the wafer
cooled down to prevent further diffusion of the silver into the
silicon.
[0028] Embodiments of the present invention add boron oxide
(B.sub.2O.sub.3) and organic fluoride salts to the nickel ink leads
to form conductive contacts between the nitride-coated silicon
substrate and the nickel once the ink is printed and fired. This
effect occurs when either the boron oxide or the fluoride salt, is
used alone or a mixture thereof. Embodiments for organic fluoride
salts used are tetramethylammonium fluoride (TMF) and
tetraethylammonium fluoride (TEF). The amount of these materials
needed to achieve conductivity may be as low as 0.1% of the solids
content. All these additives completely dissolve in the nickel ink.
In embodiments, ink formulations containing both boron oxide and
fluoride salt performed better than each component alone.
[0029] Control experiments utilizing nickel inks not containing the
above additives tailed to form conductive contacts When used on
nitride-coated silicon wafers.
[0030] The mechanism by which the foregoing additives assist
penetration through the nitride layer may involve the additives
reacting with the nitride layer to form low melting point
intermediate species which in turn, allow penetration and diffusion
of the metal through the molten layer.
[0031] Unlike the glass frits that have been widely used as fine
dispersions in solar pastes to achieve burnthrough, the
configuration of the foregoing additives to completely dissolve in
common organic solvents enables their incorporation into low
viscosity inks without the previously noted concerns associated
with particle size, particle dispersion, and settling.
[0032] Based on these results, other oxides and fluorocompounds may
express similar results, either alone or as combination of several
precursors. Examples may be phenyl boronic acid, which decomposes
to boron oxide on firing, bismuth-salycilic acid complex,
tetraalkylphosphonium fluorides, and fluorinated polymers which may
decompose on heating to provide fluoride ions.
[0033] In an example of aspects of the present invention, nickel
particles having diameters of 20 nanometers (e.g., commercially
available from Mitsui Mining and Smelting Co., Ltd., Japan) were
used to produce nickel ink formulations. Benzyl alcohol and
diethyleneglycol monobutyl ether were used as solvents. Disperbyk
111. (e.g., commercially available from Byk Gardener, Columbia
Md.). a phosphoric acid polyester, was used as a wetting agent.
Thin solar wafers coated with 70 nm of silicon nitride were used as
substrates. The nickel inks were printed (e.g., using an inkjet
printer) to print a series of lines (e.g., having widths of 35
microns and lengths of 0.5 cm). The printed wafers were dried
(e.g., 100.degree. C. for 40 minutes). After this time, the
specimens were heated in a tube furnace under a gas mixture (e.g.,
10% hydrogen in nitrogen, also known as forming gas, at
approximately 500.degree. C for 30 minutes). Additional samples
were fired as low as 300.degree. C. and a contact was formed
through the silicon nitride. Samples were also fired as high as
700.degree. C. and a contact was formed. After firing, the
specimens were tested (e.g., using a four point probe method). The
resistance between lines located at different distances was
measured. Plotting the resistance as a function of the distance
yielded a straight line. The intercept of the line, divided by two,
and multiplied by the lines' area provided the contact resistance.
More information on this test method can be found in the textbook
"Semiconductor Material and Device Characterization" by D. K.
Schroder, Wiley Interscience 2006.
[0034] Table 1 describes the nickel ink formulations tested.
Formulation A served as a control and contained nickel
nanoparticles, benzylalcohol and diethyleneglycol monobutyl ether
as sol eats, and Byk111 as a dispersant. Formulations B and C were
essentially control formulation A modified with boron oxide and
TEF, respectively. Formulations D and E contained both boron oxide
and TEF. The total percentage of boron oxide and TEF in formulation
D was 1.45%. In formulation E, the total percentage of boron oxide
and TEF was 0.03%, which is about two orders of magnitude lower.
Formulation used TMF instead of TEF in the same order of magnitude
as in formulation E. The contact resistance values achieved for
each formulation is shown at the bottom of each column in Table
1.
[0035] Formulation A, which did not contain the additives disclosed
herein that react with the silicon nitride layer, did not,
effectively form a conductive contact. Formulation D contained much
more boron oxide and fluoride salt than formulations E and F, yet
the contact resistance values were very similar for these three
pastes. Formulations B and C, containing either boron oxide or
fluoride salt, showed higher resistance (less conductivity) than
formulations C-F containing both boron oxide and fluoride salt.
TABLE-US-00001 TABLE 1 Nickel Ink Formulations (% weight) tested
and Contact resistance achieved FORMULATION A B C D E F Nickel
16.00 15.76 15.55 15.67 15.90 15.89 nanoparticles Benzyl 72.00
71.82 70.83 71.39 72.42 72.39 alcohol Diethylene- 11.00 10.51 10.37
10.45 10.60 10.59 glycol monobutyl ether Disperbyk 1.00 1.05 1.04
1.04 1.06 1.06 111 Boron oxide -- 0.86 -- 0.46 0.01 0.01 TEF -- --
2.21 0.99 0.02 -- TMF -- -- -- -- -- 0.05 Total (% wt) 100 100 100
100 100 100 Contact >1 .times. 10.sup.6 2.3 1.7 0.21 0.32 0.11
Resistance (ohm cm.sup.2)
* * * * *